X-ray imaging is important in many technical fields including medical applications.
Medical imaging is the process by which physicians evaluate an area of the subject's body that is not externally visible. Medical imaging may be clinically motivated, seeking to diagnose and examine disease in specific human patients. Alternatively, it may be used by researchers in order to understand processes in living organisms. Many of the techniques developed for medical imaging also have scientific and industrial applications.
In the case of radiography, the probe is an X-ray beam which is absorbed at different rates in different tissue types such as bone, muscle and fat. After having propagated through the body of the object under examination, the transmitted X-ray beam generates an intensity pattern being indicative of the internal structure of the object under examination.
Currently, most solid-state digital X-ray detectors in the market are built of a flat glass plate with amorphous silicon (a-Si) thin film electronics and an X-ray conversion layer on top of it. They are either of the indirect conversion type with a scintillator on top of an array of photodiodes or of the direct conversion type using a photoconductor on top of an array of electrodes. The impinging X-rays are absorbed in the conversion layer and, via the generated charges in each pixel of the array, create a digital image of the X-ray absorption.
An alternative to thin film electronics on glass is the use of wafers of monocrystalline silicon for the pixel electronics. As above, pixels with or without photodiodes can be built for either indirect or direct X-ray conversion. The use of standard CMOS processes in monocrystalline silicon leads in general to electronic circuits with less noise and more functionality compared to a-Si pixel circuits. In case of an indirect conversion detector, the scintillator can either be glued or grown directly on the Si wafer. For direct X-ray conversion materials there are also two possibilities: either connecting a separately fabricated layer, for example with bumb balls or a direct deposition on silicon. An example of a CMOS detector of the indirect conversion type can be found in H. Mori, R. Kyuushima, K. Fujita, M. Honda, “High Resolution and High Sensitivity CMOS PANEL SENSORS for X-ray”, IEEE 2001 Nuclear Science Symposium Conference Record, Vol. 1, pp. 29-33 (2001).
Furthermore, reference is made to W. Zhao, G. DeCrescenzo, J. A. Rowlands, “Investigation of lag and ghosting in amorphous selenium flat-panel x-ray detectors”, SPIE Medical Imaging Vol. 4682, pp. 9-20, 2002.
However, conventional detectors may have a sensitivity which may be insufficient for specific applications.